1. Field of the Invention
The present invention relates to an image pickup apparatus such as a single-lens reflex digital camera, and particularly to an image pickup apparatus having a function of detecting an object in a produced image.
2. Description of the Related Art
Image pickup apparatuses including the above-mentioned one are provided with an image capturing system configured to produce a displaying/recording image by using output from a main image sensor that photoelectrically converts an object image to be captured which is an optical image formed by light from an image taking optical system. Moreover, some of such image pickup apparatuses are provided with a detection system configured to produce a photometric image by using output from a sub image sensor that photoelectrically converts a photometric object image which is formed by the light from the image taking optical system. The apparatuses acquire object luminance information from the photometric image.
Single-lens reflex digital cameras have a mirror movable into and out from an optical path from an image taking optical system to switch its state between a state where light from the image taking optical system is introduced to the detection system and a state where the light therefrom is introduced to the image capturing system.
The image pickup apparatuses having such a detection system can use, as the sub image sensor, an image sensor enabling production of a photometric image having a certain level of resolution enabling object recognition. Using such a photometric image makes it possible to perform not only photometry, but also (a) searching for (tracking of) a specific object, such as a person's face, whose position and luminance vary and (b) controls such as automatic proper exposure setting and automatic maintenance of an in-focus state of the image taking optical system.
However, increase of the resolution (pixel number) of the sub image sensor narrows a dynamic range of its each pixel, which may make it impossible to perform good photometry with respect to objects with various luminances. Therefore, in order to expand the dynamic range particularly on a low luminance side, so-called pixel binning is proposed which adds output values of plural pixels and treats its result as an output value of one pixel. Japanese Patent Laid-Open No. 2006-217355 discloses an image pickup apparatus that selects whether or not to perform such pixel binning according to moving speed and luminance (light intensity) of an object.
However, the pixel binning decreases the resolution of the photometric image produced with the sub image sensor, which may decrease accuracy of the object searching using this photometric image and thereby may cause the image pickup apparatus to perform the above-mentioned controls for an incorrect object. Thus, an over-exposure image, an under-exposure image or an out-of-focus image may be produced as the displaying/recording image.